This invention relates to wheel speed sensors and more particularly relates to a method of compensating inaccuracies in a wheel speed sensor.
A method of this type is disclosed in German patent application No. 44 09 846. In this application, an accurate value of the wheel rotational speed is required to conclude different wheel diameters and, under certain circumstances, pressure loss in individual wheels from variations of the wheel speeds of the individual wheels of a vehicle. The correction factors are established by determining the wheel speed from a full turn of the vehicle wheel under defined driving conditions which mainly include that no vehicle acceleration in the longitudinal or transverse direction can be detected. Subsequently, the correction factors of these individual partial intervals of the signal-forming elements are so adjusted that the wheel speeds, which are established due to the individual signal-forming elements in consideration of the correction factors, correspond to the wheel speed which resulted in a full turn of the wheel.
An object of the present invention is to improve the updating of the correction factors.
According to the present invention, this object is achieved by directly measuring non-corrected wheel speed and comparing it with a reference value of the wheel speed established by means of a calculating model, and updating the correction factor is updated dependence on this comparison.
Compared to the prior art method, it is favorable that the correction factors are updated considerably more quickly during driving than is the case in the prior art method. While in the prior art method updating is only possible when the defined driving conditions prevail, the method of the present invention permits updating in the current driving operation irrespective of what driving conditions are currently detected. On the one hand, the correction factors depend on manufacturing tolerances of the pole wheels and related differences in the distances of the individual signal-forming elements which should be equidistant in the optimal case. This concerns uneven tooth widths and tooth gap widths and, respectively, in active sensors, uneven distances of the magnets of the pole wheel of the sensor due to manufacturing tolerances. Further, the correction factors can be affected by deformations of the signal-forming elements which may occur during driving and due to the fact that metal chips may possibly gather in the interspaces between the signal forming elements. Likewise, corrosion of the signal-forming elements can be caused. Thus, uneven tooth and tooth gap widths or pulse periods of the pole wheel due to corrosion and/or contamination and mechanical damages are referred to. The last mentioned influences cause slow or sudden variations in the wheel rotational speed which should be taken into account as instantaneously as possible by updating the correction factors in order to have available in driving correct wheel rotational speed values as quickly as possible. This is important inasmuch as the wheel speed is the measured quantity for various vehicle safety systems. The signal-forming elements can be components of passive and active sensors.
Thus, the quicker updating of the correction factors achieved by the method of the present invention has favorable effects.
To minimize the effect of only brief variations of the measurement values on the correction factors, updating of the correction factors can be effected by producing the new correction factors in averaging the previously established correction factors and the current values. For example, this may be done by a recursive least-squares method, advantageously with a forgetting factor. It is achieved by the forgetting factor that xe2x80x98oldxe2x80x99 values will no longer be taken into account after a defined period of time.
A first set of correction factors is acquired before the installation of the wheel speed sensor into a vehicle and is memorized in a control unit in the method of the present invention.
Favorably, this provides for a set of correction factors which can be updated correspondingly, simultaneously with the start of the first travel.
Preferably, the absolute position of the wheel is determined by way of a comparison of the sequence of the memorized correction factors with the sequence of currently established correction factors. This position can principally be newly determined every time a signal-forming element was detected. It is e.g. known from the state of the art to omit one tooth in a pole wheel and to identify the absolute position from the fact that when this location is passed by, the period between two signals is twice as long as the period between the other signals. In contrast thereto, no marking is necessary in the method of the present invention.
In a preferred method, the correction factors are stored one after the other, and the correction factor relating to each element is read out of the table in the determination of the wheel speed signal. When the end of the table is reached, the next correction factor at the beginning of the table is read out, and the elements are allocated to the correction factors by way of the determined absolute position.
Advantageously, the absolute position can be determined by way of the sequence of the established correction factors, and subsequently the correction factors can be read out of the table accordingly in a consecutive manner. It is possible to adjust the absolute position in the meantime. For example, this can be effected cyclically or when it is determined that the read-out correction factors differ by more than a defined amount from the currently established correction factors.
To determine the current position of the sensor wheel, the cross correlation of the established correction factors with the memorized correction factors is determined, and the absolute position of the sensor wheel is determined from value xcfx84 which corresponds to the maximum of the cross correlation. This is a relatively simple mathematical method which permits determining the absolute position of the pole wheel.